Lithium ion secondary batteries have excellent properties such as high energy density, high operation voltage and small degradation after charge/discharge cycles, and thus are widely used for portable information terminals such as cellular phones and laptop computers, and in recent years, further used for storage batteries for electric vehicles and stationary storage batteries (storage batteries for power storage). For these portable information terminals, storage batteries for electric vehicles, and stationary storage batteries, a lithium ion battery having a high capacity and being capable of high-speed charging/discharging is required.
Regarding achievement of a higher capacity, Si-based and Sn-based negative electrode materials, which have high capacity per unit weight, are considered to be promising as an alternative negative electrode material to graphite materials, and in particular Si-based negative electrode materials are advantageous also in terms of cost reduction in the future because the resource is abundant. However, such negative electrode materials suffer from difficulty in achieving high-speed charging/discharging due to low conductivity, and have problems such as a large volume variation after repeated charging/discharging.
Regarding improvement of the high-speed charging/discharging characteristics, use of a carbon material excellent in rate characteristics such as graphitizable carbon and non-graphitizable carbon for a negative electrode active material in place of graphite materials has been proposed. However, such materials have a problem of larger lowering of capacity than in the case of graphite materials when being used for a negative electrode active material.
In addition, lowering of the resistance in an electrode with a conductive aid or the like is examined to improve the high-speed charging/discharging characteristics. Examples of conductive aids include acetylene black, Ketjen black, furnace black, carbon fibers, and carbon nanotubes.
Patent Literature 1 (JP07-230803A) discloses a non-graphitizable carbon in which the pore inlet size in the surface of a fine particle is a size such that a lithium ion in an electrolytic solution of a lithium secondary battery can pass through the pore and an organic solvent in an electrolytic solution cannot pass through the pore substantially, for the purpose of enhancing the capacity of a lithium ion secondary battery.
Patent Literature 2 (JP2007-250469A) discloses a negative electrode active material including amorphous carbon having a three-dimensionally interconnected through-hole with an average through-width of 0.05 to 5 μm and a lithium ion secondary battery using it, for the purpose of providing a negative electrode active material having an excellent occluding/releasing capacity for ions.
Patent Literature 3 (JP2012-164638A) discloses a lithium ion battery using a negative electrode, the negative electrode including: a negative electrode layer containing an active material including an amorphous carbon particle capable of occluding and releasing a lithium ion; and a binder, and including a pore having a pore size of 1 nm or larger and 3 nm or smaller and a pore having a pore size of 20 nm or larger and 100 nm or smaller at a specific ration, for the purpose of enhancing the power.
Non Patent Literature 1 discloses a catalyst with a platinum particle homogeneously dispersed and supported on a single-wall carbon nanohorn, and describes application thereof to an electrode for polymer electrolyte fuel batteries.